1. Field of the Invention
This invention relates to an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus having a plurality of developing means and capable of forming color images.
2. Related Background Art
U.S. Pat. No. 4,712,906 discloses an image forming apparatus in which a plurality of developing means are disposed around a photosensitive member (that is, an image bearing member) along the direction of rotation thereof, and a plurality of transfer mediums of a size which permits a plurality of sheets to be carried on a transfer medium carrying member (transfer drum) are carried on the transfer drum at the same time to improve the number of image sheets output per unit time.
In this image forming apparatus, a charger, four developing devices of cyan, magenta, yellow and black, respectively, a transfer drum and a cleaner are disposed around a photosensitive belt. The diameter of the transfer drum is such that the transfer drum can carry two transfer mediums of the letter size thereon and form images thereon.
A cyan toner image, a magenta toner image and a yellow toner image each intended for two transfer mediums are successively formed on the photosensitive belt. The cyan toner image, the magenta toner image and the yellow toner image are transferred onto the two transfer mediums carried on the transfer drum. The toner images of the three colors are transferred onto the first and second transfer mediums carried on the transfer drum, whereafter the transfer mediums are separated one by one from the transfer drum and are sent to a fixing device.
In the above-described image forming apparatus, the interval between the two transfer mediums of the letter size carried on the transfer drum is smaller than the interval between the developing devices. Therefore, if three or more image sheets are output on end, there will be a time period in which a plurality of developing devices perform the developing operation at the same time.
When two developing devices are driven at the same time as described above, the torque required of a drive motor becomes great because the developing devices are generally driven by a single drive motor. Also, a developing bias must be applied to the two developing devices at the same time. Further, there is a drawback in that images are disturbed by the fluctuation of load.